Internal combustion engines of vehicles may have operating modes which may be selectable by the driver. Thus in some vehicles an economy mode, a normal mode and a sport mode may be provided, each mode having a different engine response to one or more commands of the driver. Typically the engine may respond differently in each mode to a given input of the accelerator pedal, being least responsive in economy mode, and most responsive in sport mode. In this way driveability of the vehicle can be improved by providing a range of accelerator pedal movement which is appropriate to, for example, the desired output torque characteristic of the engine. Such a system necessarily relies upon an electronic input from the vehicle driver, for example an accelerator potentiometer providing an input signal to an electronic control unit having a plurality of accelerator positions/output torque maps.
Another kind of operating mode relates to the terrain which a vehicle is intended to cross. U.S. Pat. No. 7,349,776, the content of which is hereby incorporated by reference, describes a vehicle control system in which the driver can implement improved control over a broad range of driving conditions, and in particular over a number of different terrains which may be encountered when driving off-road. In response to a driver input command relating to the terrain, the vehicle control system is selected to operate in one of a number of different driving modes including one or more terrain response (TR) modes. For each TR mode, the various vehicle subsystems are operated in a manner appropriate to the corresponding terrain.
In one arrangement, a mode (for example a winter mode) is available in which the vehicle is configured to launch from standstill in a forward gear other than first gear such as a second gear to reduce risk of excessive wheel slip. Different modes may have different accelerator pedal maps (amount of engine torque developed for a given accelerator pedal position), torque delivery (accelerator pedal maps in combination with a rate at which engagement of transmission clutches is controlled to take place thereby determining how aggressively gear shifts take place), and transmission shift points as a function of coefficient of surface friction. For example, in one or more TR modes the transmission shift points may be arranged wherein gear shifts take place at lower speeds than they might otherwise take place at, and in a more gentle manner (e.g. at a slower rate).
Thus one or more accelerator/torque maps appropriate to the terrain may be selectable by the driver. For example when driving in rocky terrain, a high torque output may be indicated for a small accelerator movement, thus giving immediate urge to overcome a rock step. In contrast on sand, a low torque output may be indicated for the same accelerator movement, so as to avoid spinning the vehicle wheels and digging a hole. To some extent the selected torque map may be a matter of judgment related to the available grip on the terrain.
The two extreme torque maps associated with any vehicle operating mode coincide at zero accelerator position/zero torque and maximum accelerator position/maximum torque. In between these conditions, a change of map from one (a ‘source’ map) to another (a ‘target’ map) causes an immediate change of engine output torque which may be disconcerting to the vehicle driver, especially if the accelerator pedal is not being moved at the time of change.
If an alternative operating mode is selected by the vehicle driver, a change in the characteristic of engine response is generally not a surprise—the change is in fact expected by the driver and is generally desirable. However difficulties may arise if the operating mode is automatically selected in response to a vehicle sensing a change of operating condition. Thus, for example a vehicle may detect a change of terrain from rock to sand and, whilst the accelerator is partly applied, command the vehicle engine to adopt a different torque map. The consequent change in engine response may be disconcerting to the driver, especially if such automatic mode changes are repeated frequently.
FIG. 1 shows a known motor vehicle 101 having a powertrain 101P. The powertrain 101P includes an engine 121, a transmission 124, a power take-off unit (PTU) 137, a rear driveshaft or propshaft 131R and a front driveshaft or propshaft 131F. The rear driveshaft 131R is operable to drive a pair of rear wheels 113, 114 via a rear differential 135R whilst the front driveshaft 131F is operable to drive a pair of front wheels 111, 112 via a front differential 135F.
The vehicle 101 has an engine controller 121C arranged to receive an accelerator pedal position signal from an accelerator pedal 161 and a brakes controller 141C operable to receive a brake pedal position signal from a brake pedal 163.
In the configuration of FIG. 1 the transmission 124 is releasably connectable to the rear driveshaft 131R by means of the power transfer unit (PTU) 137, allowing selectable two wheel drive or four wheel drive operation.
The PTU 137 is also operable in a ‘high ratio’ or a ‘low ratio’ configuration, in which a gear ratio between an input shaft and an output shaft thereof is selected to be a high or low ratio. The high ratio configuration is suitable for general on-road or ‘on-highway’ operations whilst the low ratio configuration is more suitable for negotiating certain off-road terrain conditions and other low speed applications such as towing.
The vehicle 101 has a central controller 101C, referred to as a vehicle control unit (VCU) 101C. The VCU 101C receives and outputs a plurality of signals to and from various sensors and subsystems provided on the vehicle 101.
The vehicle 101 has a transmission mode selector dial 124S operable to select a required operating mode of the transmission 124. The selector dial 124S provides a control signal to a transmission controller 124C which in turn controls the transmission 124 to operate according to the selected mode. Available modes include a park mode, a reverse mode and a drive mode.
The vehicle 101 also has a terrain response mode selector dial 128S. The terrain response mode selector dial 128S is operable by a driver to select a required terrain response mode of operation of the vehicle.
It is to be understood that if a user selects the drive mode of the transmission 124, the engine controller 121C employs a drive mode throttle map to determine the amount of drive torque that the engine 121 should produce as a function of accelerator pedal position. If the user selects a ‘dynamic’ TR mode, the engine controller 121C employs a sport mode accelerator (or throttle) pedal progression map instead of the drive mode accelerator pedal progression map. The throttle maps differ in that the sport mode throttle map is arranged to provide a more aggressive response by the engine 121C to a given initial advance (such as depression) of the accelerator pedal 161.
Different throttle maps are also employed for different respective user-selectable terrain response modes.
As noted above, in some arrangements the vehicle may be operable automatically to select an appropriate TR mode for the prevailing driving conditions.
FIG. 2 shows two different accelerator pedal progression maps in the form of a plot of engine torque output T as a function of accelerator pedal position P on a scale from 0 to 100% of full scale depression of the accelerator pedal 161. Two extreme vehicle operating modes A and B are illustrated. Mode A is a cautious torque map and may correspond for example to a TR mode suitable for use when driving over sand. Mode B is a more aggressive torque map and may correspond to a TR mode suitable for use when driving over rock. A driver may select operation according to mode A or mode B my means of the TR mode selector dial 128S. At the zero and 100% accelerator positions, the torque maps coincide, but at part depression of the accelerator pedal significant differences in torque output are apparent.
Thus a switch from mode A to mode B at point C (50% of application of accelerator pedal 161) results in an immediate jump to point D, with consequent increase in engine torque output. The characteristic of line B is subsequently followed. A corresponding switch in the reverse direction results in significant drop in output torque. Changes between torque maps generally comprise movements in the direction of the y axis.
The change in output torque of the engine 121 may take time, and can be deliberately blended, as illustrated in FIG. 3. Thus the increase from point C to point D may be controlled to avoid a step change. For example a maximum blending rate, say 7 Nm/s may be applied, and/or blending may take place at a defined rate within a maximum time period, of say 20 seconds. A small torque change will blend relatively quickly, and a large torque change will take longer.
It will be appreciated that for a given accelerator pedal position a change in torque delivered by the engine 121 will occur as blending progresses from one torque characteristic to another. This change may be disconcerting if the change of operating mode is determined automatically by the VCU 101C rather than the driver. It would be desirable to indicate to the driver that such a change is in progress, but visual or audible indicators are not considered desirable because the driver may be overloaded with information, and not immediately appreciate what the indicator is indicating. Furthermore automatic mode changes may occur frequently, and such indicators may be a distraction.
A mode change may also be disconcerting if occurring automatically during deceleration of a vehicle 101 to a halt. At the next acceleration, the vehicle 101 may respond quite differently to driver expectation of a given movement of the accelerator pedal 161.
What is required is an improved means and method of allowing the vehicle driver to become accustomed to a change of engine torque characteristic.
It is against this background that the present invention has been conceived. Aspects and embodiments of the invention may provide a method, a system or a vehicle which address or obviate the above problems. Other aims and advantages of the invention will become apparent from the following description, claims and drawings.